1. Field of the Invention
The present invention relates to diffractive optical elements, and more particularly to layered (laminated) diffractive optical elements, as well as optical systems and optical apparatuses using the same.
2. Description of the Related Art
As methods for reducing the chromatic aberration of lens systems, there are methods of combining glass materials, but there is also the method of providing a diffractive optical element having a diffraction effect on a portion of the optical system.
Such methods have been proposed or disclosed for example in such documents as SPIE Vol. 1354 International Lens Design Conference (1990), in Japanese Patent Laid-Open No. 1992-213421 (corresponding to U.S. Pat. No. 5,044,706), Japanese Patent Laid-Open No. 1994-324262 (corresponding to U.S. Pat. No. 5,790,321), and U.S. Pat. No. 5,044,706.
Methods using diffractive optical elements utilize the physical phenomenon that chromatic aberration at refractive surfaces and diffractive surfaces occurs in opposite directions with respect to light rays of a certain reference wavelength.
Furthermore, diffractive optical elements can be provided with an aspheric lens effect by appropriately changing the period of their periodic structure, so that they are also effective for reducing other aberrations besides chromatic aberration.
In lens systems having a diffractive optical element, when light rays of the used wavelength region are concentrated to diffracted light of one specific order (referred to as “specific order” or “design order” in the following), then the intensity of the diffracted light of other diffraction orders becomes low, and if their intensity is zero, diffraction light thereof is not present at all.
However, if there is diffracted light of an order other than the design order, and if it has a certain intensity, then it is formed images to a different location than the light of the design order, so that it becomes flare light in the optical system.
Consequently, in order to utilize the diffractive optical element's effect of reducing aberrations, the diffraction efficiency of the diffracted light of the design order needs to be sufficiently high for the entire used wavelength region, and it is important to adequately consider the spectral distribution of the diffraction efficiency for the design order as well as the behavior of the diffracted light of orders other than the design order.
FIG. 16 shows a diffractive optical element made of a substrate 302 and a diffraction grating 301 formed on the substrate 302 (referred to as “single-layer DOE” in the following). The characteristics of the diffraction efficiency for the specific order when this single-layer DOE is formed on a certain surface are shown in FIG. 17.
In FIG. 17, the horizontal axis marks the wavelength of incident light, and the vertical axis marks the diffraction efficiency (this is also the same in all other figures illustrating diffraction efficiency). The values of the diffraction efficiency are the ratios of the light amount of diffracted light at each order with respect to the light amount of the entire transmitted light, and light reflected at grating boundary surfaces is not considered, as it would only complicate the explanations.
As shown in FIG. 17, the single-layer DOE shown in FIG. 16 is designed such that the diffraction efficiency becomes highest at the used wavelength region for the first diffraction order (bold solid line in FIG. 17), so the design order is the first order. At this design order, the diffraction efficiency becomes highest at a certain wavelength (below, this wavelength is referred to as the “design wavelength”), and gradually becomes lower at other wavelengths. Diffraction light at other orders increases by the same rate as the diffraction efficiency at the design order decreases, and this diffraction light at other orders becomes flare light. FIG. 17 also shows the diffraction efficiency of other orders near the design order (namely zero-th order and second order, which are the orders plus or minus 1 of the first order (design order)).
The following has been proposed as a scheme for reducing the influence of the flare light that is produced like this.
With the diffractive optical element proposed in Japanese Patent Laid-Open No. 1997-127322 (corresponding to U.S. Pat. No. 6,157,488), a somewhat higher diffraction efficiency can be achieved for the design order across the entire visible wavelength region, as shown in FIG. 19, as shown in FIG. 18, by optimally selecting three different grating materials 306 to 308 and two different grating thicknesses d1 and d2, and adhering the three diffraction gratings together with identical grating pitch distribution.
Furthermore, as shown in FIG. 13, the diffractive optical element proposed in Japanese Patent Laid-Open No. 2000-98118 (corresponding to the U.S. patent application Publication Ser. No. 2001/0015848A1) is a diffractive optical element having a structure in which element portions 202 and 203 that respectively include a single-layer DOE are placed in proximity to one another sandwiching an air layer 210 between them (in the following, diffractive optical layers with such a structure are referred to as “layered DOEs”). In this layered DOE, by optimizing the refractive index, dispersion characteristics (Abbe number νd) and grating thickness of each layer of the materials constituting each single-layer DOE, a high diffraction efficiency can be achieved for the design order across the entire visible wavelength spectrum, as shown in FIG. 14.
Furthermore, by prescribing the Abbe numbers of the materials constituting the diffraction grating, a high diffraction efficiency is achieved with grating thicknesses of 10 μm or less. Correspondingly, as shown in FIG. 15, also the diffraction efficiencies of the diffraction orders plus or minus 1 of the design order can be better suppressed than with the single-layer DOE of FIG. 17.
By using the diffractive optical elements proposed in the above-noted Japanese Patent Laid-Open No. 1997-127322 and Japanese Patent Laid-Open No. 2000-98118, the diffraction efficiency of the design order is improved greatly compared to single-layer DOEs, attaining a high diffraction efficiency of at least 94% over the entire used wavelength region, and at least 98% in the main wavelength region of 450 nm to 650 nm. Moreover, flare light of unnecessary diffraction orders is favorably reduced to 2% or less over the entire used wavelength region, and 0.6% or less in the main wavelength region of 450 nm to 650 nm.
For this reason, when applied to optical systems in which the image-taking or projection conditions do not change (such as reader lenses of copying machines or projection lenses of liquid crystal projectors), the influence of flare can be suppressed with single-lens DOEs to a level at which it is not a problem.
However, in optical systems of optical apparatuses that take a variety of object images at a variety of conditions, such as still cameras or video cameras, even tiny amounts of remaining flare may become a problem.
For example, if there is a light source in the objects, then image-taking is generally performed not such that the light source is properly exposed, but such that the object other than the light source is suitably exposed.
For this reason, the image of the light source within the objects is taken at an exposure above the suitable exposure. For example, if the light source is exposed at 500 times the suitable exposure, then even with a flare of barely 0.6% remaining, the flare of the light source is multiplied by 500 times:0.6×500=300%,thus becoming three times the flare of the suitable exposure, and will definitely show up in the taken image.
Thus, if a layered DOE is applied to an optical system of a still camera or a video camera, even a tiny amount of flare may become a problem.